Stargazing: What is the Twilight Zone?

Even if you’re not into stargazing, you’ve heard of the Twilight Zone. Is it real? Yes, but not in a spooky way. Just after the Sun sets in the evening or just before it rises in the morning, the sky is still lit up – a time known as twilight. This illumination is because the Sun can still shine its light on the atmosphere above you, even if it can’t shine its light directly on you.

How much twilight you get depends on where on Earth you are: stargazers near the equator have far shorter twilights than those at higher latitudes.

Twilight actually comes in three types:

Civil twilight: Civil twilight is what most people mean when they talk about twilight. It starts in the morning when the Sun is six degrees below the horizon, and ends at sunrise. In the evening, civil twilight starts at sunset and continues until the Sun is six degrees below the horizon. During civil twilight, the sky is still bright enough that, in general, you don’t need lights when doing things outside.

Nautical twilight: Nautical twilight is when the Sun is between 6 and 12 degrees below the horizon. During nautical twilight, you can still distinguish the sky from the distant horizon when at sea, which allows sailors to take measurements of bright stars against the horizon (hence the name). Most people consider this time dark, but it’s still technically twilight.

Astronomical twilight: Astronomical twilight is when the Sun is between 12 and 18 degrees below the horizon. During astronomical twilight, you can no longer tell the sky from the distant horizon when at sea, but crucially for stargazers, light is still in the sky.

Not much light appears – indeed, you’d probably say it’s properly dark at this point – but the faintest objects in the sky, such as nebulae and very dim stars, will be visible only after astronomical twilight ends. Also, any long-exposure photographs of the night sky may show up this twilight that your eyes may miss.